This is now supported in YAML Schema.
Microsoft Variable Group Documentation
For quick reference, from their documentation the usage looks like:
variables:
- group: my-variable-group
- name: my-bare-variable
value: 'value of my-bare-variable'
I came across this when looking for support for setting the variable group name dynamically such as: -group: connectioninfo-$(env)
but this is currently not supported. The issue on this is located here in case this enhancement to variables groups is of interest to you in the future as well.
https://github.com/MicrosoftDocs/vsts-docs/issues/3702